Hollow cylindrical sleeves are often employed in flexographic printing as a means of quickly mounting and dismounting plates on imaging cylinders, printing press cylinders, storage cylinders, and other cylinder-based equipment. For various reasons, it is generally desirable to avoid touching the surface of the un-processed media normally mounted on the sleeve's outer cylindrical surface. Avoiding touching the media surface is particularly difficult while the sleeve is being handled during mounting and dismounting from the cylinder. With the more recent desire to image the media directly on the sleeve, the care required in handling the sleeve is even more critical.
Typically, the sleeves used in flexographic printing are tubular in form and made of composite, polymer, or metal. Sleeves are commonly affixed to the printing cylinder using an interference fit i.e. the free-state circumference of the sleeve's interior surface is less than or equal to the circumference of the exterior surface of the cylinder. The requirement for an interference fit presents problems related to the mounting and dismounting of sleeves from the cylinder.
A common method for mounting and dismounting sleeves is described in U.S. Pat. No. 4,903,597 to Hoage at. al. Hoage et al. disclose introducing fluid pressure between the exterior cylindrical surface of the cylinder and the interior cylindrical surface of the sleeve. This method is schematically depicted in FIG. 1. Cylinder 11 is supported in a cantilevered condition with a free end 11a ready to receive a sleeve 12. The proximate end 12a of a sleeve 12 is initially forced onto free end 11a of a cylinder 11. After the sleeve 12 has been pushed a short way onto cylinder 11, a fluid pressure is established via small apertures 13 in the exterior surface of cylinder 11. The fluid pressure (commonly pressurized air) exerts radial force on the sleeve 12, expanding it slightly and creating an air bearing on which the sleeve 12 may be slid onto the cylinder 11. When the sleeve 12 is in the desired location relative to the cylinder 11, the fluid pressure is removed and the sleeve 12 shrinks to its regular size, forming an interference fit on the cylinder 11. It is advantageous if the apertures 13 are located relatively close to the free end 11a of the cylinder 11, so that the fluid pressure may be applied in the region where the sleeve initially engages the free end 11a of cylinder 11.
There are some problems with this method. One such problem is that the proximate end 12a of the sleeve 12 must initially be forced onto the free end 11a of cylinder 11 before the air bearing created by the apertures 13 can overcome the interference between the sleeve 12 and cylinder 11. This initial mounting requires accurate axial alignment of sleeve 12 and cylinder 11. Any binding will make it difficult to push sleeve 12 far enough onto cylinder 11 to allow the air bearing to form. As mentioned earlier, the media (not shown) on the exterior surface of the sleeve 12 is sensitive and may not be physically manipulated during this procedure. Even if the sleeve 12 is only manipulated by its distal end 12a, there is a significant chance that such manipulation will damage the media on the sleeve's external surface, or damage the sleeve itself.
On completion of imaging the sleeve is usually removed from the imaging cylinder. An air bearing is once again established via apertures 13. The only way to remove sleeve 12 from cylinder 11 is to apply a force to the very thin edge of its proximate end 12a. This can damage the printing media, the image carried on the media, or even cylinder 11. Another problem occurs when the proximate end 12a of the sleeve 12 gets closer to the free end 11a of the cylinder 11a during the removal process. In this condition a substantial portion of the sleeve 12 overhangs the cylinder 11, which may cause the sleeve 12 to bind or lock on the cylinder 11. If the sleeve 12 binds on the cylinder 11, one may need to push the sleeve 12 back onto the cylinder 11 and try to remove it again. Obviously, such remounting and dismounting substantially increases the amount of manipulation of (and potential damage to) the sleeve 12 and the sensitive media on its outer surface.
There remains a need for better apparatus and methods for mounting and dismounting sleeves on cylinders.